In farm implements, depth control of the working tools in the soil is important to success of the operation. In the past, some implements, such as cultivators, were used primarily for weed control where precise tool depth in the soil was important but not critical. More recently with increasing emphasis on minimal tillage techniques, the requirement for precise depth control of the working tools has become critical. The tools of such implements are typically positioned between raised and lowered positions by a master/slave hydraulic system incorporating multiple cylinders located at intervals across the implement, which can be up to about thirty-five (35) feet or more in width. Such systems generally incorporate a single master cylinder which in turn controls several slave cylinders connected in series to the master cylinder, which in turn is controlled by a reversible flow control, center detent valve on the tractor supplying the operating pressure.
"Rebound", however, has been a long-standing obstacle to achieving precise depth control in agricultural implements with the master/slave hydraulic systems of the prior art. Rebound occurs from the expansion of the various hoses, fittings, fluid and other components during the process of lowering the tools to the desired working depth, which is usually set by means of a hydraulic stroke control or mechanical stop at the master cylinder. When the stop is engaged, the hydraulic pressure in the system continues to build causing a slight expansion of the system components. When this pressure is relieved, which typically occurs through leakage across the reversible control valve, the slave cylinders become slightly displaced by the "extra" hydraulic fluid as these components relax. This displacement is cumulative depending upon the distance of the slave cylinder from the master cylinder. For example, in a three-section implement utilizing one master cylinder and three slave cylinders, variance between positioning of the slave cylinders can range up to about three-eighths (3/8) of an inch. In a five-section implement incorporating one master cylinder and five slave cylinders, positioning of the rods of the slave cylinders can vary up to about three-fourths (3/4) of an inch across the implement. This of course makes it extremely difficult to maintain precise depth control to the degree required in modern farming techniques, particularly minimum tillage seedbed preparation wherein consistent placement of the chemical, seed and fertilizer at precise depths is required.
Various attempts have been made to solve the problem of rebound in such master/slave hydraulic systems. For example, one major manufacturer of farm implements utilizes two master cylinders, one for controlling a series of slave cylinders on each side of the implement. This is relatively expensive and reduces but does not eliminate rebound of the slave cylinders, although the rebound is the same between corresponding slave cylinders on opposite sides of the implement. Another approach has been simply to reduce pressure in the return line with a pressure relief valve, however, this results in uncontrolled flow, thus dumping system pressure, which also affects operation of the control detent valve. Others have used a check valve to lock the system at a high pressure which aggravates rebound, and results in complete dumping of pressure if the operator manipulates the control detent valve for adjustment. The most common approach to the rebound problem has been to make the necessary adjustments to the cylinders in the field, however, this is not efficient or practical. Otherwise, the problem of rebound has heretofore defied solution. Some have characterized rebound as an inherent problem in such master/slave hydraulic systems.
There is thus a need for an improved master/slave hydraulic system which eliminates rebound and is thus adapted for use on agricultural implements requiring precise depth control.